The frequency conversion device includes first of all a first pass-band filter for filtering radio-frequency signals picked up by an antenna of the receiver, said first filter being a passive selective filter for eliminating image frequencies during a first frequency conversion. The device further includes means for generating oscillating signals for generating first and second high frequency signals, the frequency of the first signals being higher than the frequency of the second signals. The first high frequency signals are mixed with the filtered radio-frequency signals in a first mixer unit. The signals generated by the first mixer unit have a frequency equal to the result of a subtraction of the frequency of the first signals from a carrier frequency of the radio-frequency signals. The device further includes a second pass-band filter for filtering the signals coming from the first mixer unit and a second mixer unit for mixing the signals filtered by the second filter with the second high frequency signals. The signals generated by the second mixer unit have a frequency equal to the result of a subtraction of the frequency of the second signals from the frequency of the signals coming from the first mixer unit. Finally, the device includes means for shaping the signals provided by the second mixer unit for generating the intermediate signals.
In the case of a GPS receiver, the correlation stage has the task of extracting the GPS messages or data from the intermediate signals received from the device. The messages are transmitted to microprocessor means of the receiver for calculating the position and time-related data. Of course, the receiver has to pick up the radio-frequency signals of at least four visible satellites for the position calculation.
The frequency conversion device can also be used in any radio-frequency signal receiver other than a GPS receiver. It may for example be a receiver used in a satellite navigation system of the GLONASS or GALILEO type. It may also be a receiver used in a mobile telephone network, for example of the CDMA type (Code-division multiple access).
The use of RF receivers, particularly of the GPS type, is currently widespread. This allows a user of such a receiver to be able to take a bearing in the direction of a desired target and to find the position of his current location. Consequently, it becomes necessary to be able to incorporate an RF receiver in objects which are used daily and which a single person can easily carry.
GPS receivers can be mounted for example in a wristwatch or a mobile telephone. However, in order to be mounted in these small sized objects, the receivers have to fulfil certain conditions. On the one hand, the power consumption of low power receivers has to be greatly reduced, since the objects are powered by batteries or accumulators of small size. On the other hand, the number of components of the receiver also has to be considerably reduced.
Usually, the frequency conversion devices of RF receivers are designed to perform a triple frequency conversion of the received radio-frequency signals. An embodiment of such a device according to the prior art is shown schematically in FIG. 1.
With reference to FIG. 1, the frequency conversion device is connected to an antenna 2 of the RF receiver, particularly of the GPS type, for picking up RF signals originating from visible satellites. For civil applications, the carrier frequency of the GPS radio-frequency signals has a value of 1.57542 GHz.
The radio-frequency signals picked up by the antenna are first of all filtered and amplified by a first filtering and amplification element 101. The radio-frequency signals filtered by element 101 are then mixed in a first mixer 102 with first high frequency signals provided by a voltage-controlled oscillator 109. The signals thus generated by mixer 102 are signals whose frequency is equal to the result of a subtraction of the frequency of the first high frequency signals from the carrier frequency of the filtered radio-frequency signals.
For this first frequency conversion operation by first mixer 102, the pass-band filter of element 101 has to be a selective passive filter of the SAW type. Said filter of element 101 has to be sufficiently selective to eliminate the image frequency of the radio-frequency signals at the input of the first mixer.
With a frequency of the first high frequency signals equal, for example, to 1.3961 GHz, the frequency of the signals generated by the first mixer has a value of approximately 179.3 MHZ. Thus the first selective filter has to be capable of eliminating the image frequency having a value of 1.2168 GHz (1.3961 GHz-0.1793 GHz) from the received radio-frequency signals.
The signals generated by the first mixer 102 are filtered and amplified by a second filtering and amplification element 103. The signals filtered by element 103 are then mixed in a second mixer 104 with second high frequency signals provided by a first divider 110 connected to oscillator 109. The signals thus generated by second mixer 104 are signals whose frequency is equal to the result of a subtraction of the frequency of the second high frequency signals from the frequency of the signals generated by the first mixer.
For this second frequency conversion operation by second mixer 104, the pass-band filter of element 103 also has to be a selective passive filter in particular of the SAW type.
The first high frequency signals are divided, for example, by 8 using a first divider 110 for generating the second high frequency signals whose frequency has a value of approximately 174.5 MHz. Thus, the frequency of the signals generated by the second mixer has a value of approximately 4.8 MHz. The second selective filter thus has to be capable of eliminating the image frequency having a value of approximately 169.7 MHz (174.5 MHZ-4.8 MHZ) of the signals generated by first mixer 102.
The signals generated by second mixer 104 are then filtered and amplified by a third filtering and amplification element 105 which includes a pass-band filter. The signals filtered by third element 105 are mixed in a third mixer 106 with clock signals CLK provided by a frequency divider 115 connected to a reference oscillator 114.
The frequency of the reference signals generated by the reference oscillator has a value, for example, of 17.452 MHz. This reference frequency is divided by 4 by divider 115 to generate clock signals at the frequency of 4,363 MHz. Thus for this third conversion operation, the frequency of the signals generated by the third mixer is close to 400 kHz.
The signals generated by third mixer 106 still have to be filtered and amplified by a fourth filtering and amplification element 107, which includes a low-pass filter, and be sampled and quantified by a sample and hold converter 108. This converter 108 is clocked by clock signals CLK.
In order to generate the first and second high frequency signals, the device has a phase lock loop frequency synthesiser 100. This synthesiser includes voltage controlled oscillator 109, two frequency dividers 110 and 111, a frequency and phase detector 112 for comparing the frequency of the signals originating from oscillator 109, which are divided by dividers 110 and 111, with the frequency of the reference signals generated by reference oscillator 114. The control signals leaving detector 112 are filtered by a low-pass filter 113 in order to generate a control voltage at oscillator 109 as a function of the comparison of the signals provided to said detector 112.
A major drawback of the device of FIG. 1 is that it includes too large a number of electronic components to achieve the triple frequency conversion. A large part of the components still has to operate at high frequency. Consequently, the current consumption of the device is too high. One cannot therefore envisage mounting an RF receiver comprising the device in an object of reduced size, since said object includes a battery or accumulator of small size. This battery or accumulator would be too quickly discharged during operation of the RF receiver.
Another drawback lies in the use of at least two selective pass-band filters of the SAW type, which are expensive and bulky components. The dimensions of each encapsulated filter is of the order of 5 mm×5 mm×1.3 mm, which involves a significant loss of space for assembly in an object of small size such as a wristwatch or a cellular telephone. It should also be noted that SAW type selective filters generate a gain loss of the filtered signals which means that said signals have to be amplified for the subsequent processing operations.
In order to reduce the number of components for the frequency conversion in such a device, it has already been proposed to perform only a double frequency conversion instead, of the usual triple frequency conversion. European Patent No. EP 0 523 938 B1, which discloses a radio receiver, can be cited in this regard. Said receiver includes a frequency conversion device performing a double frequency conversion of the radio-frequency signals received at an antenna.
The frequency conversion device shown in FIG. 1 of this Patent includes a phase lock loop frequency synthesiser 30 formed mainly of a voltage controlled oscillator 28, which provides first high frequency signals to a first mixer 14. The first mixer also receives radio-frequency signals filtered and amplified by amplification and filtering element 12, which includes a selective pass-band filter. The converted signals generated by the first mixer have a frequency of the order of 200 MHz.
A second amplification and filtering element 16, which includes a selective pass-band filter, filters and amplifies the signals generated by first mixer 14 in order to provide filtered signals to a second mixer 18. This second mixer 18 also receives second high frequency signals whose frequency is an integer number times less than the frequency of the first high frequency signals. The frequency of the signals produced by the second mixer is approximately equal to 26 MHz. These signals and then filtered and amplified by a third element 20 before being provided to a processor 22.
One drawback of the device presented in this European Patent lies in the fact that it is necessary to use two selective passive filters, which can, for example, be of the SAW type. Even if the two mixers and a part of the frequency synthesiser are integrated in the same integrated circuit, the two selective filters cannot, however, be integrated in said integrated circuit. Consequently, a significant loss of space remains with the use of external filters, and the cost of making a device with these expensive components remains high.
Another drawback lies in the fact that the current consumption remains high, since a large part of the components operates at high frequency and the selective filters consume an enormous amount. Assembly of such a receiver in an object of small size cannot be easily achieved.
An object of the present invention consists in making a frequency conversion device that reduces energy consumption, as well as the number and size of electronic components as much as possible to carry out a double frequency conversion in order to overcome the drawbacks of the devices of the prior art. Thus, the RF receiver including said device can easily be mounted in an object of small size, such as a wristwatch or a cellular telephone.